Printed circuit board assemblies are used in computers, communications equipment, televisions, and many other products. In a typical printed circuit board assembly, many electrical components are attached to the top and bottom surfaces of a printed circuit board (PCB). Since the electronics manufacturing industry is highly competitive, it is important to maximize the through-put of processing PCB assemblies and to securely attach the electrical components to the PCBs.
The manufacturing of PCB assemblies involves many processes, one of which is surface mounting components to PCBs. To surface mount components to a first surface of a PCB, a dispenser deposits solder pads and/or adhesive pads onto the first surface of the PCB, and then a pick-and-place machine presses the components against the solder/adhesive pads. After the first side of the PCB has been populated with components, the PCB is inverted and the process is repeated to surface mount components to the second side of the PCB. The solder/adhesive dispenser is typically a stenciling machine, and the pick-and-place machine is typically a turret-type device.
Manufacturing issues related to supporting the populated sides of PCB assemblies are best understood in light of the structure and operation of conventional stencil printing machines. Conventional stenciling machines generally have a stencil plate, a movable platform under the stencil plate, and a PCB holder attached to the platform. Conventional stenciling machines also have two spaced-apart, parallel conveyor tracks to transport the PCB along a conveyor line between the stencil plate and the movable platform. In operation, the conveyor tracks engage opposing edges of the PCB and transport the PCB until it is over the PCB holder. The moveable platform then moves upwardly and engages the PCB holder with the downwardly facing side of the PCB to lift the PCB to the stencil plate. A wiper then moves across the stencil plate to press the solder paste or the adhesive compound onto the PCB. After the wiper stops, the work platform moves downwardly from the stencil plate to replace the PCB on the conveyor rails so that it can be transported to a pick-and-place machine.
In addition to maximizing the through-put of processing PCB assemblies, it is also becoming important to accurately mount a large number (e.g., 1000-1500) of very small components to one side of the PCB assemblies. An important aspect of surface mounting processes, therefore, is to accurately deposit very small solder pads and adhesive pads at precise locations on the PCB. Another important objective of surface mounting processes is to accurately press the electrical components onto the pads at the precise locations so that the electrical terminals of the components engage the desired solder pads. The PCB holders must accordingly hold the PCB assemblies flat with respect to the stencil plate so that a desired volume of the solder/adhesive compound is deposited in each pad across the surface of the PCB. Similarly, the PCB holders must hold the PCB assemblies flat in pick-and-place machines so that the terminals of the components engage the desired contacts on the surface of the PCB. Thus, it is generally desirable to uniformly support the underside of the PCB so that it does not bow or dip.
To uniformly support the underside of PCBs, one type of a conventional PCB holder has a platform and a number of pins removably attached to the platform to engage open areas or components on the underside of a PCB assembly. The pins of this type of a conventional PCB holder generally have a fixed height, and thus to engage a specific component, a pin having the correct height to support the specific component must be mounted to the platform at a location corresponding to that of the specific component. Another type of conventional PCB holder may have a platform, sidewalls projecting up from the platform to support the perimeter of a PCB, and spacers positioned within the sidewalls to support interior portions of the PCB. It will be appreciated that the pins and the spacers of conventional PCB holders generally prevent the PCBs from bowing to hold the PCBs flat for surface mounting processing.
Conventional PCB holders, however, encounter several manufacturing issues when components are surface mounted to both surfaces of a PCB assembly. For example, one problem with conventional PCB holders is that the pins or spacers may not be at the appropriate height or location to uniformly support the populated side of the PCB. The pins and spacers of conventional PCB holders are configured manually to engage vacant areas or to have a height that accommodates the components on the populated side of the PCB. However, it is extremely time-consuming to manually configure the individual supports on the platform to engage the vacant areas on a PCB. Moreover, it is even more difficult to select and position pins with the appropriate heights to engage components on the populated side of the PCB. Conventional PCB holders, therefore, may not uniformly support the PCB assemblies during surface mounting processes.
Conventional PCB holders are particularly difficult to operate efficiently in the manufacturing of contract PCB assemblies in which multiple runs of different PCB assemblies with different configurations of components are processed in a short period of time. Unlike dedicated PCB assembly manufacturing in which a large number of identical PCB assemblies are manufactured in each run, the down-time associated with configuring PCB holders in contract PCB manufacturing may significantly increase the cost of manufacturing the PCB assemblies. Therefore, there is a significant need to quickly and consistently support PCB assemblies with different configurations of components in the manufacturing of contract PCB assemblies.